1. Field of the Invention
The invention concerns a process and a device for measuring the electrical brain activity and, in particular, evaluation of the measurement results.
2. Description of the Related Art
In medicine, one is confronted by the problem of measuring the acute phase functional capacity of the human brain after the occurrence of, for example, a stroke.
In 1910, Berger discovered that the electrical activity of the brain could be measured as an electroencephalogram (EEG) by using sensitive amplifiers on the human scalp. Brain damage such as, for example, circulatory perturbations, frequently manifest themselves in alterations in the EEG; that is, changes in the signal frequencies or amplitudes. There is locus specificity; that is, resolution of the exact location of the damage site in the brain in contrast with imaging procedures such as, for example, computer tomography of the cranium, is very restricted. On the other hand, the temporal resolution of brain activity or alterations in activity is substantially better. In addition, the full extent of the brain insult can be seen only after passage of hours or days in tomography, by which time the tissue is irreparably damaged.
Measurement of an EEG is, for example, presented in the U.S. Pat. Nos. 5,392,788 and 5,269,315 documents.
The system described in U.S. Pat. No. 5,392,788 is used to test the patient""s reaction to stimuli. In this instance, the data are processed after acquisition so that they can be compared to baseline values. Deviations between data and baseline values are used to determine irregularities.
U.S. Pat. No. 5,269,315 describes a process and a device for the interpretation of brain currents, whereby certain brain activities are combined in a primary frequency range and in a second frequency range. After application of a Fourier transformation to the digitally measured data the data is further processed by segregating the entire output range obtained by the Fourier transformation into an absolute output and a relative output.
U.S. Pat. No. 5,458,117 describes a system and a process for derivation of a diagnostic index from 19 measured EEG signals. At evaluation of the signals, for example, a fast Fourier transformation or a cross-correlation is applied to all signals measured. The determination of the diagnostic index is relatively costly due to the many signals that must be measured.
Consequently, to date neither purely clinical methods nor imaging processes, such as nuclear spin tomography, are adequate for objective determination of the extent of brain damage in the acute phase, since there is no simple answer to the question, whether the symptoms presenting in the patient are caused by a circulatory perturbation at all. A much higher outlay for equipment is required in imaging procedures and in the acute phase; that is, in the few hours following the appearance of the symptoms, they do not allow a detailed determination to be made of the extent of the damage. However, since it is particularly important in the acute phase that the correct therapeutic decisions are made, there is considerable interest in rapid and uncomplicated diagnostic methods.
DE-OS 19 61 593 describes the performance of a comparison of EEG activity of the hemispheres. In this instance, the selection of electrode placement is similar to that in the present invention. A correlative coefficient indicates in a specific time whether the two channels being examined exhibit the same polarity. If, in addition, the amplitudes of the signals of the analysis are added, as is indicated (see 5.17) in DE-OS 19 61 593 using the Pearson""s Product-Moment-Correlation Coefficient, the value of the correlation function of the two channels is determined at a fixed time lag [lit. xe2x80x9cshiftxe2x80x9d] At (here up to 100 bit at 1 kHz=0.1 sec). The correlation coefficient indicates the instantaneous polarity behavior; no temporal track is calculated or temporal averaging done. Moreover, in the procedure described in DE-OS 19 61 593 so-called xe2x80x9cevoked potentialsxe2x80x9d are measured with priority; this means that the patient is subjected to a stimulus (most often visual), whereupon the electrical cortical response is measured.
U.S. Pat. No. 4,412,547 describes an apparatus for monitoring brain activity, whereby the measurement process is accomplished with few EEG electrodes. In the subsequent analysis of the signals measured, however, only very rough characteristics of the EEG signal are compared: the outputs of the individual channels (if required, after frequency filtering) and the xe2x80x9caverage frequencyxe2x80x9d determined using a disputable process (xe2x80x9czero crossingxe2x80x9d) for such a complex signal. In the process described in U.S. Pat. No. 4,412,547 a complex mix of frequencies is converted into a single frequency display or indication.
Starting with the device described in U.S. Pat. No. 269,315, the invention takes up the problem of creating a device, by means of which the physician can rapidly obtain a decision-criterion at examination of a patient apparently affected by stroke, that will allow the most certain prognosis of the extent of the brain damage and its future development.
Pursuant to this invention, very detailed properties of the measured electrical signals are used. Thus a sample analysis done on the FFT of the individual channels and the FFT of the correlation function relevant to its peak structure (location, height and breadth). The structural properties that are then calculated are compared in order to ultimately determine a derived magnitude, namely the brain function index, that provides for a simpler interpretation. In particular, pursuant to the invention, an analysis of the frequency structure of the entire correlation function is done.
With the process described in the invention for measuring electrical brain signals, particularly in humans, a quick and effective analysis of the measured signals can be performed and they can be digitized, stored and subsequently analyzed on a computer.
The device described in the invention makes possible stepwise processing of the brain waves measured on the patient, whereby it is particularly advantageous, that the procedure can be performed in conjunction with other clinical diagnostic procedures.
The invention provides a simple, clear interpretation of the results as well as easy operation, for example, by nursing staff. In addition, the level of error security is very favorable.
In the following, the invention is described in detail using realization examples with reference to the illustration.